In determining the condition of an internal combustion engine, it is important among other things to determine the condition of the piston rings by determining how tightly the piston rings fit against the cylinder wall. A measure of the condition of the piston rings can be obtained by measuring the pressure produced in the combustion chamber after several compression strokes of the piston. The greater the pressure obtained with a given number of compression strokes of the piston, the better the condition of the rings.
In order to obtain a measure of the compression pressure produced by the piston, it has been customary to remove the spark plug from the cylinder and insert a tightly fitting probe which senses the pressure within the cylinder. This probe communicates the pressure in the cylinder to a pressure measuring device which can indicate the pressure on a dial or make a record of the pressure. In one device, a dial indicator arm moves a marking pin over a card to mark the card and indicate the pressure in the cylinder. Each time a test is completed for a particular engine, it is necessary to change the card in the device before testing the next engine. This procedure is time consuming and it is not possible to record the pressures developed on more than one engine on a given card for comparison with other engines. One type of pressure measuring device consists of a cylinder containing a piston with sealing rings. The rings produce considerable friction against a cylinder wall which interferes with the sensitivity of the device. In another case, the pressure sensitive device consists of a bourdon type tube in which the pressure indicating arm is connected to one end of the tube and the arm moves a linkage to position a pointer. The bourdon tube is awkward to handle and does not provide a compact measuring device which can be held in the hand of the operator while making measurements in the vicinity of the engine.